WELDING RESEARCH JUNE 2016 / WELDING JOURNAL 203-s produce the molten droplets and weld pool continuously. However, the metal transfer as well as welding arc burning process occurs in the periodically changing bubbles in water. Consequently, the welding area becomes a very complex system with intense interactions in the welding arc, metal transfer, and bubbles. Understanding the system elements, behaviors, and interaction mechanisms is fundamental for improving the process stability and weld-joint quality. The mechanisms of interaction between underwater arc plasma and the water environment have been studied based on spectroscopic analysis and simulation. Based on the Damkohler principle and Stark broadening, Pan et al. demonstrated that underwater plasma flew at equal rates and had sufficient electron density; namely, it was in local thermodynamic equilibrium under certain conditions (Ref. 10). This certified the validity of spectroscopic analysis on underwater plasma. For underwater manual metal arc welding, Wang et al. found the temperature of the underwater arc was lower than that in air, which was thought to be caused by the cooling effect from continuously generated, growing, and bursting bubbles (Ref. 11). Li et al. found that under increasing pressures the densities of H, H+, O, C, O+, C+ in bubbles increased as well, but the average ionization degrees were not influenced (Ref. 12). Furthermore, the authors verified the existence of H in the underwater plasma, and also deduced that in either environment of air and water, the arc plasma was mainly composed of self-shielding gas and evaporated metals with only minor effects stemming from the interaction with water (Ref. 13). Tsai et al. investigated the rapid cooling mechanisms in water, and the bubbles’ dynamics were simulated to determine the boundary conditions, concluding that the rapid cooling was mainly caused by the surface heat conduction behind the arc (Ref. 14). Ghadimi et al. investigated the effect of the material, surrounding fluid, and the method of heat losses through modeling and analyzing, showing that the convective heat transfer is more effective than the radiation, which therefore can be neglected (Ref. 15). Subsequently, the research by Liu and Olson showed a typical wet-welding procedure reportedly had t8/5 values between 1 and 6 s (in contrast with the air welding of 8 to 16 s), and the greater cooling rate produced significant amounts of heat-affected zone (HAZ) martensite in nearly all lowcarbon steels (Ref. 3). As such, additional induction heating was applied to reduce the cooling rate of the joint in underwater wet welding for a higher quality weld joint (Ref. 16). These indirect proofs are not sufficient for investigating the internal physical mechanisms of droplets, arc, and bubbles. Visual sensors have been recognized as one of the most effective methods for studying metal transfer and arc behaviors during consumable arc welding. Kim and Eagar (Ref. 17) used high-speed videography to study the metal transfer modes’ transition during conventional GMAW. Furthermore, Shi et al. defined four metal transfer modes for the dual-bypass GMAW based on images with 0.5-ms intervals between each frame (Ref. 18). As far as the underwater welding was concerned, Prof. Madatov et al. studied the wet welding method (1.2- mm-diameter wire) based on an x-ray high-speed camera in 1965, and found that the droplet size was about 2–3 times that of the wire (Ref. 19). Guo et al. also used the x-ray transmission method to monitor the metal transfer and spatters during underwater FCAW (Ref. 20). With the same method, Fu and Feng et al. (Ref. 21) defined the classification of the metal transfer modes, including globular repelled, surface tension, explosive short-circuit, and submerged arc transfer (from 237 to 440 A). However, the bubbles and welding arc are generally difficult to recognize in the research papers due to the inherent physic principle of the x-ray transmission. Conventional CCD cameras were employed to monitor the weld pool by Shi (Ref. 22) and to sense the welding arc behavior during underwater FCAW by Liu (Ref. 23). Although the weld pool edge and welding arc profile were extracted with limited resolution, the details of welding arc, droplets, and Fig. 1 — Schematic of the image capture system for underwater wet FCAW. Fig. 2 — Spectral radiation curve during underwater wet FCAW.
Welding Journal | June 2016
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